A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein, or a liquid with a nano-particle suspension (e.g. particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable include a hydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueous solution.
The manufacture of ICs and other devices with such apparatus generally involves the replication of extremely fine sub-micron patterns, with an exceptionally high degree of positional accuracy. For this reason, it is essential to properly isolate various critical parts of the apparatus such as the substrate table and support structure (i.e. mask table), for example, from spurious motion, vibration, mechanical shocks, etc. In general, this is achieved using such measures as carefully designed metrology frames, air-mounts, motional counterweights and dampers, which serve to isolate the critical parts of the apparatus from most unwanted mechanical influences. However, such measures are not completely effective in eliminating a number of unwanted influences, such as, for example:
1. vibrations in the substrate table due to leveling actions during exposure;
2. vibrations caused by motion of reticle masking blades;
3. resonance effects caused by the presence of air showers;
4. immersion forces caused by water flow between the projection system and the substrate;
5. vibrations in the substrate table caused by motion of the support structure, and vice versa; and
6. influence of air shower flow on the substrate table. Although these effects are relatively small, they become increasingly important as the need to produce ever-higher device resolutions increases, and they now form a substantial barrier to the viable realization of large-area ICs having critical dimensions of the order of 0.15 μm and less.
Accordingly, it has been proposed in EP-0 967 525-A to provide a control system for the substrate table and support structure of a lithographic apparatus in which errors in the position of the substrate table are compensated for by their inclusion as a feed-forward control in the support structure control loop. Specifically, the substrate table error is lowpass filtered, the output of the filter is then added to the support structure setpoint, and also twice differentiated, multiplied by the support structure mass and the resultant force applied to the support structure. This proposal is based on the realization that the absolute positions of the support structure and substrate table are less important than their relative position and allows the correction of substrate table errors beyond the support structure bandwidth. However, this control system has performance limits, in part caused by the inevitable time delay in processing the substrate table error.
It has been proposed in EP-1 265 104-A to provide a control system for the mask table (patterning means) and substrate table that predicts the momentary substrate table position error and feeds it into the mask table control loop, adding it to the mask table set point and as a force to the mask table.